Carbonaceous materials are commonly used as reinforcing materials for plastics, metals and carbon because of their mechanical high strength and high elastic modulus. In addition, because of their high crystallographic and mechanical anisotropy, they are also useful as lubricating materials for sliding parts. Their unique microstructures are also utilized for applications in capacitor batteries and lithium secondary batteries. However, carbonaceous materials have a drawback in that they undergo degradation at temperatures of a few hundred .degree. C. and above by oxidation in the presence of oxygen and other oxidative gases, or react with the metals or other matrixes to produce carbides, so that their expected performance cannot be exhibited; applications of carbonaceous materials as reinforcing materials and lubricating materials and so on have therefore been limited.
Boron nitride has also been applied as a sliding material, etc. Boron nitride is a chemically stable material with high oxidation resistance, but is difficult in control of its crystallinity and form than carbonaceous materials.
It has been attempted to utilize the similarities and differences between carbonaceous materials and boron nitride to produce materials which are combinations thereof. Japanese Unexamined Patent Publication (Kokai) No. 8-245206, for example, discloses a boron carbonitride material which is an intermediate compound of graphite and boron nitride wherein carbon, nitrogen and boron are in a molar composition of 6:2:1. Also, Japanese Unexamined Patent Publication (Kokai) No. 5-171444 discloses a material with excellent wear resistance comprising a bi-layer film composed of a hard carbon film and a boron nitride film which is formed on a coated substrate by vapor phase synthesis. Japanese Unexamined Patent Publication (Kokai) No. 7-187883 discloses a process for coating a silicon substrate with a thin film of boron nitride alloyed with carbon, which can be employed for wear resistant uses and for semiconductors across a wide range of temperatures. The processes for forming these boron nitride films involve vapor phase reaction under reduced pressure, but while such processes are effective for producing films on flat surfaces, they are difficult for materials with not flat surfaces, such as carbonaceous particles and carbonaceous fibers.
Carbonaceous particles and carbonaceous fibers containing boron atoms and nitrogen atoms are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 3-245458, No. 5-266880, No. 5-290843, No. 8-306359 and No. 8-31422. These carbonaceous particles and carbonaceous fibers, however, have the boron atoms or nitrogen atoms in solid solution in the carbon, or have carbon atoms replaced with boron atoms or nitrogen atoms, or are stoichiometrically simple compounds of carbon, nitrogen and boron.
Graphite is also used as a negative electrode active material in lithium secondary batteries, and Japanese Unexamined Patent Publication (Kokai) No. 9-63584 discloses the use of a carbon material which has been subjected to graphite treatment in the presence of a boron compound as a negative electrode active material in a lithium secondary battery, in order to achieve increased activity for the negative electrode active material. However, graphitization treatments in the presence of a boron compound it does not make possible to obtain a graphite material coated with boron nitride, and no improvement in chemical stability is achieved.
It is an object of the present invention to modify the chemically unstable surfaces of conventional carbonaceous particles and carbonaceous fibers by coating them with a boron nitride thin film, to thus provide carbonaceous particles and carbonaceous fibers with a widened range of applications.
It is another object of the invention to provide lithium secondary batteries with improved chemical stability of their negative electrode active materials by using the aforementioned boron nitride thin film-coated carbonaceous particles and carbonaceous fibers as the negative electrode active materials.